1. Field of the Invention
The present invention relates to an apparatus and method for driving a plasma display panel, and more particularly, to an apparatus and method for driving a plasma display panel in which a gray scale inversion phenomenon can be prevented.
2. Description of the Background Art
A plasma display panel (hereinafter, referred to as PDP) is a display device that employs the principle that a visible ray is generated from phosphors when the phosphors are excited with a vacuum ultraviolet generated upon discharge of a gas. The PDP is advantageous in that it is thin in thickness and light in weight and can be made large with high definition, compared to a cathode ray tube (CRT) that has become the main stream of a display means so far. The PDP is composed of a number of discharge cells arranged in a matrix shape, and one of the discharge cells constitutes one pixel.
FIG. 1 is a perspective view illustrating the structure of a discharge cell of a conventional three-electrode AC surface discharge type PDP.
Referring to FIG. 1, the discharge cell of the conventional three-electrode AC surface discharge type PDP includes a scan electrode 12Y and a sustain electrode 12Z both of which are formed on the bottom surface of an upper substrate 10, and an address electrode 20X formed on the top surface of a lower substrate 18.
An upper dielectric layer 14 and a protection film 16 are laminated on the upper substrate 10 in which the scan electrode 12Y and the sustain electrode 12Z are formed parallel to each other. Wall charges generated upon plasma discharge are accumulated on the upper dielectric layer 14. The protection film 16 serves to prevent damage of the upper dielectric layer 14 due to sputtering generated upon the plasma discharge, and improve efficiency of secondary electron emission. Magnesium oxide (MgO) is typically used as the protective layer 16.
A lower dielectric layer 22 and barrier ribs 24 are sequentially formed on the lower substrate 18 in which the address electrode 20X is formed. A phosphor layer 26 is coated on the lower dielectric layer 22 and the barrier ribs 24. The address electrode 20X is formed in a direction in which the address electrode 20X cross the scan electrode 12Y and the sustain electrode 12Z.
The barrier ribs 24 are formed parallel to the address electrode 20X, and serve to prevent ultraviolet and a visible ray generated by a gas discharge from leaking toward neighboring discharge cells. The phosphor layer 26 is light-emitted by ultraviolet generated upon plasma discharge to generate one of red, green and blue visible rays. An inert gas for the gas discharge is injected into discharge spaces defined between the upper substrate 10 and the barrier ribs 24 and between the lower substrate 18 and the barrier ribs 24.
This PDP is driven with one frame being divided into several sub-fields having a different number of discharges in order to represent the gray scale of an image. Each of the sub fields is subdivided into a reset period for generating a uniform discharge, an address period for selecting a discharge cell, and a sustain period in which the gray scale is represented depending on the number of a discharge.
For example, if it is desired to represent an image with 256 gray scales, a frame period (16.67 ms) corresponding to 1/60 seconds is divided into eight sub-fields SF1 to SF8, as shown in FIG. 2. Furthermore, each of the eight sub-fields SF1 to SF8 is subdivided into the reset period, the address period and the sustain period. In this time, the reset period and the address period of each of the sub-fields are the same every sub-field, but the sustain period of each of the sub-fields increases in the ratio of 2n (n=0, 1, 2, 3, 4, 5, 6, 7) in each sub-field.
FIG. 3 is a waveform shown to explain a method of driving the conventional three-electrode AC surface discharge type PDP.
Referring to FIG. 3, one sub-field is divided into a reset period where the whole screen is initialized, an address period where data is written while scanning the whole screen in the progressive scan mode, and a sustain period where cells into which data is written keep light-emitted.
In the reset period, a reset waveform RP is applied to scan electrode lines Y1 to Ym at the same time. If the reset waveform RP is applied to the scan electrode lines Y1 to Ym, a reset discharge is generated between the scan electrode lines Y1 to Ym and sustain electrode lines Z1 to Zm, so that discharge cells are initialized.
In the address period, a scan pulse SP is sequentially applied to the scan electrode lines Y1 to Ym. A data pulse Dp, which is synchronized to the scan pulse SP, is applied to the address electrode lines X1 to Xn. In this time, an address discharge is generated in discharge cells to which the data pulse Dp and the scan pulse SP are applied.
In the sustain period, first and second sustain pulses SUSPy, SUSPz are alternately applied to the scan electrode lines Y1 to Ym and sustain electrode lines Z1 to Zm. In this time, a sustain discharge is generated in discharge cells in which the address discharge is generated.
In this PDP, the brightness is determined according to the following Equation 1.
                              B          graylevel                =                  gain          ⁢                                          ⁢                                    Q              k                                      i              =              1                                ⁢                                          ⁢                      A            i                    ⁢                      N            i                    ⁢          s                                    (        1        )            
In the above equation, B is the brightness, A is sub-field mapping information, k is the number of a sub-field, N is sub-field weight, and s is once discharge brightness of a sustain pulse.
Furthermore, gain is obtained by using the ratio of the sustain number to the number of the gray scale. In other words, gain=a total number of sustain/(gray scale level 1). For example, if a total number of sustain is 255 and a total number of the gray scale is 256, gain is set to “1”.
The sub-field mapping information A indicates selecting information of an address period. For example, if a discharge cell is selected in the address period, A is set to “1”. If a discharge cell is not selected in the address period, A is set to “0”. N indicates a weight of a sub-field corresponding to the current number of a sub-field k. s designates the brightness generated by once sustain discharge.
For example, if gain is set to 1, twelve sub-fields exist, and weights of the sub-fields are respectively set to 1, 2, 4, 8, 16, 32, 32, 32, 32, 32, 32 and 32 in a PDP, the brightness of the PDP can be set as in Table 1.
TABLE 1GrayWeight of sub-fieldsBright-Scale12481632323232323232ness 0XXXXXXXXXXXX 0S 1◯XXXXXXXXXXX 1S 2X◯XXXXXXXXXX 2S. . .. . .. . .31◯◯◯◯◯XXXXXXX 31S32XXXXX◯XXXXXX 32S. . .. . .. . .255 ◯◯◯◯◯◯◯◯◯◯◯◯255S
In Table 1, “X” means that the gray scales are not represented, and “O” means that the gray scales are represented. As can be seen from Table 1, the PDP includes the twelve sub-fields, and represents 256 gray scales by using 1, 2, 4, 8, 16, 32, 32, 32, 32, 32 and 32 brightness weights.
Table 1 shows the brightness of the PDP in consideration of only light generated by the sustain discharge. In a PDP that is actually driven, however, light is generated by the reset discharge and the address discharge as well as the sustain discharge. As such, if the gray scales are represented inclusive of the reset discharge, the address discharge and the sustain discharge, a gray scale inversion phenomenon occurs, as shown in FIG. 4. In other words, there occurs a case where the brightness of a PDP represented in a n (n is a natural number)−1 gray scale is set to be brighter than those represented in a n gray scale.
This will be described in more detail. As can be seen from Table 1, in order to represent the 31 gray scales, the sub-fields having the 1, 2, 4, 8 and 16 brightness weights have to be selected. Therefore, in order to represent the 31 gray scales, an address discharge is generated in the five sub-fields. On the contrary, in order to represent the 32 gray scales, the sub-field having the 32 brightness weight must be selected. Accordingly, in order to represent the 32 gray scales, the address discharge is generated in the one sub-field. In this time, a brightness inversion phenomenon is generated because of the light generated by the address discharge between the 31 gray scale and the 32 gray scale. In other words, the 31 gray scale generates light, which is brighter than that generated by the 32 gray scale.
In reality, the brightness of a PDP including light generated in the reset discharge and the address discharge can be determined by the following Equation 2.
                                          B            graylevel                    ⁢                                          ⁢                      (                          r              ,              a              ,              s                        )                          =                  LSr          +                                                    Q                k                                            i                =                1                                      ⁢                                                  ⁢                          A              i                        ⁢            Sa                    +                      gainS            ⁢                                                  ⁢                                          Q                k                                            i                =                1                                      ⁢                                                  ⁢                          A              i                        ⁢                          SN              i                        ⁢            Ss                                              (        2        )            
In this equation, L is the number of sub-fields that are initially reset, r is once discharge brightness of a reset pulse, and a is once discharge brightness of an address pulse.
L indicates the number of sub-fields in which a reset discharge is generated. For example, if twelve sub-fields exist and the reset discharge is generated in the twelve sub-fields in a PDP, L can be set to 12.
A matrix of Equation 3 can be induced from Equation 2.

Meanwhile, in the conventional PDP, in order to stabilize the sustain discharge in the sustain period, a pair of sustain pulse is additionally applied to each sub-field.
The brightness including light generated by the pair of the sustain pulses can be determined by the following Equation 4.
                                          B            graylevel                    ⁡                      (                          r              ,              a              ,              s                        )                          =                  LSr          +                                                    Q                k                                            i                =                1                                      ⁢                                                  ⁢                          A              i                        ⁢            Sa                    +                      gainS            ⁢                                                  ⁢                                          Q                k                                            i                =                1                                      ⁢                                                  ⁢                          A              i                        ⁢                          SN              i                        ⁢            Ss                    +                                                    Q                k                                            i                =                1                                      ⁢                          A              i                        ⁢            Ss                                              (        4        )            
A matrix such as Equation 3 is induced from Equation 4. The values r, a, s can be found by using the matrix. Usually, the value r (once discharge brightness of the reset pulse) is 0.208815[cd/m2], a (once discharge brightness of the address pulse) is 0.413396[cd/m2], and s (once discharge brightness of the sustain pulse) is 0.44553[cd/m2]. In this time, the values r, a and s are not actual brightness, but are values calculated using the equation. The brightness similar to actual brightness can be obtained by substituting the values r, a and s.
The brightness of the PDP, which includes the discharge brightness of the reset pulse, the discharge brightness of the address pulse and the discharge brightness of the sustain pulse, i.e., the brightness of the PDP by Equation 4 can be expressed into the following Table 2.
TABLE 2GrayWeight of Sub-FieldScale12481632323232323232Brightness 0XXXXXXXXXXXX12r + 0a + 0s + 0s 1◯XXXXXXXXXXX12r + 1a + 1s + 1s 2X◯XXXXXXXXXX12r + 1a + 2s + 1s. . .. . .. . . 31◯◯◯◯◯XXXXXXX12r + 5a + 31s + 5s 32XXXXX◯XXXXXX12r + 1a + 32s + 1s. . .. . .. . .255◯◯◯◯◯◯◯◯◯◯◯◯12r + 12a + 255s + 12s
In Table 2, in the gray scale of 0, only the brightness of a reset pulse, which is generated in 12 sub-fields, is represented. In the gray scale of 1, the sustain brightness corresponding to the brightness weight of 1, the brightness by a pair of sustain pulses, the brightness by 12 reset pulses, and the brightness by one address discharge are represented. Furthermore, in the gray scale of 31, the sustain brightness corresponding to the 31 brightness weight, the brightness by five sustain pulse pairs, the brightness by 12 reset pulses, and the brightness by five address discharges are represented. Moreover, in the gray scale of 32, the sustain brightness corresponding to the brightness weight of 32, the brightness by one sustain pulse pair, the brightness by 12 reset pulses, and the brightness by one address discharge are represented.
In this time, if the values r, a and s are substituted in the gray scale of 31, the brightness of “20.61184” is represented in the PDP. Furthermore, if the values r, a and s are substituted in the gray scale of 32, the brightness of “17.62166” is represented in the PDP. That is, in the conventional PDP, the gray scale inversion phenomenon is generated and an image having a linear brightness cannot be represented accordingly.